1. Field of Invention
The present invention relates to fabrication of Q-switches and laser pump cavities. Specifically this invention relates to solid state devices having a radially dependent dopant valence state density.
2. Description of the Related Art
A laser is a device which produces a beam of coherent light. In a typical laser, an incoherent light source imparts energy to a lasing medium, which produces light in which the waves are in phase, termed coherent light, through particular electron transitions. Where the lasing medium is properly designed, the coherent light is emitted as a beam. In certain cases, it is desirable that the emitted beam of coherent light be more intense than naturally occurs from the lasing medium, and a type of laser termed a Q-switched, pulsed laser has been developed for this purpose.
The pulsed laser contains a light controller termed a Q-switch which limits the buildup of light reflecting back and forth within the laser resonator until it reaches some selected value, at which time the growth of the internal wave rapidly increases and the threshold for laser action is reached. After releasing the built-up, light energy as a pulse, the Q-switch recovers to its prior function of restraining the light energy until the intensity is high enough for another pulse. Very high peak powers and beam energies can be achieved in this manner. Electrical, mechanical, and passive Q-switches are known in the art.
In one type of passive Q-switch, a solid medium contains saturable absorbers. One type of saturable absorber is based on color centers. A color center is a crystal defect that absorbs light. In a passive Q-switch, the presence of the color centers causes the transmittance of the Q-switch to be low when the energy of the incident light beam is below the saturation intensity of color center absorption. On the other hand, a sufficiently high beam intensity causes the color centers to momentarily change or bleach so that the light beam is no longer absorbed.
U.S. Pat. No. 4,833,333 by S. C. Rand and assigned to Hughes Aircraft Company, entitled “Solid State Laser Q-Switch Having Radial Color Center Distribution”, appears to teach a passive Q-switch with a radially dependent distribution of saturable color centers having the highest density at the periphery of the crystal. This patent also appears to teach fabrication of such switches by irradiating the periphery of a LiF crystal rod with electrons to produce F2− color centers, with a higher density of color centers near the periphery than in the center. Rand appears to teach irradiation of a crystal with electrons from the side, or radially. Optionally, the crystal can also be irradiated axially with electrons or in any direction with another form of radiation having greater penetration into the crystal, such as gamma radiation, to establish a base level of saturable color centers throughout the crystal. The crystal, having the radial distribution of color centers, acts as a Q-switch, but additionally tends to reduce beam divergence and increase the brightness of the laser beam by discriminating against the higher order transverse modes otherwise allowed to propagate within the laser resonator. The stated advantage of this Q-switch configuration is that it tends to reduce beam divergence and increase the brightness of the laser beam by virtue of the nonlinear bleaching mechanism.
The beam inside the laser cavity must pass through the Q-switch. Unbleached color centers act to delay the time at which the laser reaches threshold by absorbing a fraction of the growing intra-cavity field on each pass. When the intensity reaches a value, termed the saturation intensity, which is characteristic of the density and type of color centers, the color centers bleach to a transparent state and the beam is transmitted as a short, giant pulse of energy.
Unfortunately, color center Q-switches have several shortcomings including (1) the need for an expensive 1–2 MeV electron irradiation source for fabrication (and possibly a Cobalt-60 source of gamma radiation to provide a background level of color centers), (2) a relatively long crystal, which is expensive and not generally suitable for small laser cavities of the type used in miniature, eye safe laser rangefinders, (3) relatively poor control of optical density resulting in variations in lasing threshold and efficiency and requiring selection of suitable devices (low production yield). Also, F2− color centers are quite photosensitive and will disappear under weak UV exposure [see W. Gellermann et al, J. Appl. Phys. 61, 1297–1303 (1987)], and the color centers are somewhat temperature sensitive making them non-ideal for fielded applications.
Crystal lasers, doped with an active ion, often use one or more flashlamps or laser diodes to provide “pump light”. The pump light excites ions in the doped crystal to a higher energy state. This process is known as “absorption”. A “pump cavity” typically contains a uniformly doped gain medium, which may be a crystal or glass or polycrystalline element fabricated in the shape of a rod, slab, or disk; and other elements, such as a pump light reflector or relay optics. Pump light is coupled into the cavity, typically with one or more flashlamps or laser diodes, either from the side of the cavity, known as “side-pumping,” or the end of the cavity, known as “end-pumping.” A laser is created by placing the doped medium and pump cavity in a “resonator” that reflects photons created by spontaneous emission, i.e. those generated by the normal decay of the excited ions, and amplified by stimulated emission. For example, mirrors placed at either end of the doped medium and aligned perpendicularly to its longitudinal axis form a laser resonator. If the resonator is properly sized and a sufficient number of photons are reflected back and forth within the resonator so that the “gain” exceeds the “loss,” a laser oscillation will build up from spontaneous stimulated emission, i.e. “lasing” will occur, producing laser light. Laser light is typically extracted from the doped medium in the pump cavity along the longitudinal axis. Pump cavities are discussed in W. Koechner, Solid-State Laser Engineering, 3rd edition, Springer Verlag (1992), ch. 6.
Efficient absorption, in which nearly all of the pump light is absorbed by the doped medium, is a primary goal of laser designers. One method of attaining efficient absorption is by using high-absorption (highly doped) laser materials. A ray of pump light going through a doped crystal one time is known as a “pass.” With most existing designs, a pump light ray makes only one or two passes through the doped crystal before escaping, necessitating the use of high-absorption materials to achieve efficient absorption. Absorption is governed by an exponential finction. Thus, when such a crystal is side-pumped, non-uniform absorption and thus non-uniform gain often result, with the highest gain being near the edge of the lasing medium. To facilitate handling, the corners of a laser rod or slab will typically be chamfered. The chamfer will shadow or block the laser light, and since the highest gain is at the edges of the crystal, inefficient lasing results.
Another approach to the goal of high efficiency absorption uses end pumping, in which pump light comes into a pump cavity along its longitudinal axis. End pumping requires expensive high-brightness pump diodes and durable, difficult-to-produce dichroic coatings since the pumping and laser light extraction take place through the same optical surfaces (i.e. the ends of the rod) while requiring quite different reflectivity characteristics. In the case of quasi-four level or three-level systems where the high threshold requires greater pumping rate, pump “bleaching” can occur, in which a large fraction of the active ions have been excited and correspondingly fewer ions are in the ground state available for pump light absorption, resulting in reduced absorption for both side- and end-pumping geometries.
The theory of operation of the passive Q-switch is described in detail in Koechner, specifically for organic dye and radiation-induced color-center saturable absorbers [Koechner, W., Solid State Laser Engineering, 2nd Ed., Springer-Verlag, Berlin, pp 437–442 (1988).]. See U.S. Pat. No. 4,833,333.
U.S. Pat. No. 5,761,233 issued Jun. 2, 1998 to Brusselbach et al., the teachings of which are incorporated herein by reference, teaches the fabrication of a multi-element “monolithic” pump cavity using the process of diffusion bonding practiced by Onyx Optics, Inc (Dublin, Calif.). This process adds significant cost and cycle time to the manufacture of solid-state lasers and is therefore undesirable, particularly in the cost-driven eye safe laser rangefinder market for individual soldier weapon fire control systems.
Hence, a need remains in the art for solid state devices, such as Q-switches and laser pump cavities, which have radially dependent dopant densities but: 1) are fabricated without the need for an expensive 1–2 MeV electron irradiation; 2) are fabricated without the need for a Cobalt-60 source of gamma radiation to provide a background level of color centers; 3) do not require a relatively long crystal; 4) have good optical density; 5) are not photosensitive; 6) are not sensitive to weak UV exposure; 7) are not temperature sensitive and 8) are inexpensive to manufacture.